Part of an electrician""s job is to chisel out holes in walls to receive wall boxes to accommodate power points, light switches and the like. These are generally formed by hand chiseling. Once the recess is made, the box is positioned and a mark made. The box is then removed and a hole drilled at the position of the mark for receiving a wall plug for the fixing screw for attaching the box to the wall. This is time consuming, particularly if a number of boxes must be fitted, for instance in a newly built house. These boxes are generally square or rectangular but may be other shapes, such as conduit boxes which are cylindrical and therefore require circular holes.
According to the present invention an attachment for a percussive tool comprises:
means for connecting the attachment to a percussive tool; and
a plurality of rows of teeth arranged substantially parallel to and spaced from each other to form a shape substantially having rotational symmetry about a central axis.
According to a second aspect of the present invention, a method for forming a hole in a wall comprises the steps of:
providing an attachment for a percussive tool having a plurality of rows of teeth arranged substantially parallel to and spaced from each other to form a shape substantially having rotational symmetry about a central axis;
connecting the attachment to a percussive tool;
placing the attachment in contact with the wall and causing percussive action of the attachment such that the rows of teeth form a plurality of parallel grooves and ridges in the wall; and
rotating the attachment through an angle about the central axis and placing the attachment in contact with the wall in a position overlaying the plurality of parallel grooves and causing percussive action of the attachment to break up the ridges and form a hole of a substantially uniform depth.
The teeth may be individual projections, but preferably the teeth are formed as saw tooth shaped projections along the edges of parallel planar chisel blades.
The parallel planar blades create a series of parallel grooves in the wall and the attachment is then rotated through an angle and applied again to break up the ridges between the grooves to remove an area of wall in the shape of the attachment. For an attachment shape having N-fold rotational symmetry, the attachment must be rotated through approximately 360xc2x0/N, or a multiple thereof, such that the outline of the second cut overlays the first, but the grooves formed by the blades cross those formed by the first cut. The debris is ejected through the spaces between the chisel blades. Means may be provided to limit the depth to which the hole is chiseled, for instance a projection may be provided on the edge of the attachment. This allows a hole of a predetermined depth to be produced. Slots may be provided in the back of the device between the chisel blades to allow debris to more easily be ejected.
The saw tooth shaped projections along each blade improve the cutting ability of the attachment and also allow the debris produced to more easily be ejected from under the attachment. They reduce the contact area of the attachment on the wall, compared to the contact area of blades having continuous edges, which allows the attachment to be used with more lightweight, less powerful tools. The spacing of the blades and the pitch of the teeth may be chosen to match the power of the tool.
The attachment of the present invention may be designed to produce holes of a variety of shapes having rotational symmetry. For instance, the parallel chisel blades may vary in length such that the whole tool has a circular formation. Rotating such an attachment through any angle less than 180xc2x0 for the second cut would result in a circular hole. For producing a hole in the shape of an equilateral triangle, the tool must have an equilateral triangular shape and the rotation must be through 120xc2x0 such that the second cut overlays the first cut. However, preferably the chisel blades are arranged to form a square and the attachment is rotated through 90xc2x0 to form a square hole.
Preferably, the attachment comprises an attachment head and a drive impact shaft. Preferably the means for connecting the attachment to a percussive tool comprises a standard fitting such as an SDS fitting, an SDS Max fitting, or similar chucks used on percussive power tools, or other means to connect to the percussive tool. Therefore, by providing drive impact shafts having a variety of standard fittings the attachment may be fitted to the chuck of almost any percussive tool such as a so called combie drill or impact breaker. Preferably the drive impact shaft is connected to the attachment head by means of a taper fit. Preferably, locking means are provided to lock the taper fit in engagement. A number 2 Morse taper angles are preferably used and the head of the tool is retained by a centralizing and locking screw. The shaft may be released by removing the locking screw and using a parallel punch of a smaller diameter than the screw thread against the base of the screwed hole in the shaft.
With the above described arrangement, the attachment head may be readily replaced when it is worn by detaching it from the drive impact shaft. Preferably, the attachment is formed from hard cast metal formed by an investment casting operation. It is therefore straightforward to manufacture and replace when worn.
Preferably, the attachment also includes a locating rod protruding from the centre of the attachment head beyond the extent of the chisel blades. Preferably the locating rod screws into the end of the drive impact shaft and provides the means for locking the taper fit. The locating rod may be used to locate the device by locating the rod in a pre-drilled hole whilst forming the first set of parallel grooves to locate the attachment as it is rotated through 90xc2x0. This therefore ensures that the second cut overlays the first and the hole is the correct shape. The rod also may serve to limit the depth of the hole chiseled when it reaches the bottom of the pre-drilled hole. The pre-drilled hole is drilled using a standard electric drill and is there to be used as a hole for a plastic wall plug for a fixing screw for fixing the box to the wall.
Often, a hole is required which is not square but rectangular, for instance a hole for a double power point. This can readily be achieved with the device of the present invention by forming a second square hole adjacent to or partly overlapping the first. Means may be provided to aid in location of the second square hole such as a template which fits into the first square hole and carries a guide for locating the pre-drilled hole for locating the second square hole. In general, double wall boxes are of the same height as the single wall boxes and therefore the same attachment may be used to form rectangular holes for double wall boxes.
The present invention is particularly suited for use in an electric hand tool capable of percussive action such as a Combie drill or impact breaker. However, it may also be used with a pneumatic percussive tool.